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Nobel Lecture

Nobel Lecture, May 2, 1903

Light Radiation in a Magnetic Field

As Professor Lorentz told you last
December, immediately after hearing of the great and very
honourable distinction awarded to us, we set to work to see how
best to co-ordinate our two lectures. To my great regret I was
unable to be present here for Professor Lorentz's lecture, but he
was able to report to you on present electron theory from his
viewpoint, only briefly touching on the experimental
investigations which have occupied me in recent years. I hope
that you will allow me therefore to emphasize these experimental
investigations. Two fields of physics, light and magnetism, are
combined in the subject of today's lecture, whose history dates
only from the days of Michael Faraday. The wonderful discovery of
the connection between light and magnetism, which he made in
1845, was the reward for an investigation carried out with
indefatigable patience and tenacity. Today we call this
connection the magnetic rotation of the polarization plane.
Faraday succeeded in showing that the plane in which light
oscillations take place, is rotated as soon as light passes
through special magnetizable bodies along the lines of force.
Faraday himself called his discovery the magnetization of light
and the illumination of magnetic lines of force. His contempories
did not understand this name, which perhaps corresponded more to
what he was searching for than to what he found. Throughout his
life his hopes, desires and yearnings led him to make repeated
investigations into the connection between light, magnetism, and
electricity.

The last experiment recorded in Faraday's
laboratory notebook and ostensibly the last in his life, gives an
indication of the extent to which his spirit was still occupied
with the boundary region of possible phenomena.

It was on March 12, 1862, in the laboratory
of the Royal Institution that Faraday carried out this
experiment. The notes in his notebook, although not quite clear,
leave no doubt that he was attempting to demonstrate by means of
a spectroscope that magnetism has a direct effect on a light
source. The result was however absolutely negative, and Faraday
writes in his notebook "not the slightest effect demonstrable
either with polarized or unpolarized light".

Perhaps it was because of this observation
that Maxwell, at a meeting of the British Association in
Liverpool on September 15, 1870, said of the light-radiating
particles in a flame "that no force in nature can alter even very
slightly either their mass or their period of oscillation", a
statement which, coming from the mouth of the founder of the
electromagnetic light theory and spoken with such intensity, must
really surprise present-day physicists.

It was not simply out of a spirit of
contradiction that I exposed a light source to magnetic forces.
The idea came to me during an investigation of the effect
discovered by Kerr on light reflected by magnetic mirrors.

When it is a question of splitting up the
light of a luminous gas into very fine detail, the simple glass
prism of Newton and Frauenhofer is of no use, and the physicist
has recourse to the excellent aid which we owe to Rowland: the
concave grating. Most physics institutes possess this polished
metal mirror with a very large number of grooves, say 50,000 over
a width of 10 cm scratched on by means of a diamond. A beam of
compound light is no longer reflected by the lined surface in the
ordinary way; instead each special kind of light follows its own
path.

Of course the light source must be very
restricted for the large number of beams corresponding to the
various kinds of light to appear separately. This is ensured by
placing the light source behind an opaque screen with a linear
slit. The spectral image produced can be observed, and from the
location and intensity of the linear-slit images we can determine
how the different kinds of light in the light being studied are
distributed on the basis of the period of oscillation and
intensity. A further main advantage of Rowland's grating is that
it is now no longer scratched on plane surfaces, but on spherical
concave surfaces with a radius of say 3 metres, so that real
images are produced of luminous lines without the need for the
insertion of lenses. Moreover, photography has made it possible
to fix these images and now provides us with a permanent record
of each observed spectrum, which can be measured out at any
time.

When we study the well-known Bunsen sodium
flame by means of Rowland's grating, we see a spectrum consisting
mainly of two separate sharp yellow lines, which in our grating
lie about I mm from each other. We see that sodium radiation
consists of two kinds of light, the periods of oscillation of
which differ only very slightly (1 in 1000) from each other. We
confined our attention exclusively to one of these two lines.

I must ask you now to go with me into the
Physics Institute of Leiden University. In August, 1896, I
exposed the sodium flame to large magnetic forces by placing it
between the poles of a strong electromagnet. Again I studied the
radiation of the flame by means of Rowland's mirror, the
observations being made in the direction perpendicular to the
lines of force. Each line, which in the absence of the effect of
the magnetic forces was very sharply defined, was now broadened.
This indicated that not only the original oscillations, but also
others with greater and again others with smaller periods of
oscillation were being radiated by the flame. The change was
however very small. In an easily produced magnetic field it
corresponded to a thirtieth of the distance between the two
sodium lines, say two tenths of an Angstrom, a unit of measure
whose name will always recall to physicists the meritorious work
done by the father of my esteemed colleague.

Had we really succeeded therefore in
altering the period of vibration, which Maxwell, as I have just
noted, held to be impossible? Or were there some disturbing
circumstances from one or more factors which distorted the
result? Several of such might be mentioned.

We doubted the result. We studied the light
source in the direction of the magnetic force, we perforated the
poles of the magnet; but even in the direction of the magnetic
lines of force we found that our result was confirmed. We also
studied the reverse phenomenon, the absorption of light in sodium
vapour, and this too satisfied our expectations. We then asked,
do different substances behave in different ways? What happens
when the magnetic force is raised to the maximum attainable
values? How do different lines of the same substance behave? But
before these questions could be answered, theory took over.

I was in fact able to verify experimentally
some conclusions which followed from the theory of optical and
electrical phenomena of my esteemed teacher and friend Professor
Lorentz. This theory assumes that all bodies contain small
electrically charged mass particles, "electrons", and that all
electrical and optical processes are based on the position and
motion of these "electrons". Light oscillations result from the
vibration of the "electrons". On the basis of Lorentz's theory, if
we limit ourselves to a single spectral line, it suffices to
assume that each atom (or molecule) contains a single moving
electron.

Now if this electron is displaced from its
equilibrium position, a force that is directly proportional to
the displacement restores it like a pendulum to its position of
rest. In this model the electrons are represented by the red
balls and the direction of the magnetic force by the arrows. Now
all oscillatory movements of such an electron can be conceived of
as being split up into force, and two circular oscillations
perpendicular to this direction rotating in opposite directions.
In the absence of a magnetic field the period of all these
oscillations is the same. But as soon as the electron is exposed
to the effect of a magnetic field, its motion changes. According
to well-known electrodynamic laws, an electron moving in a
magnetic field is acted upon by a force which runs perpendicular
to the direction of motion of the electron and to the direction
of the magnetic field, and whose magnitude is easily determined.
Here the rectilinear oscillation is not changed by the magnetic
field, the period remains the same; on the other hand the two
circular oscillations are subjected to new forces which, running
parallel with the radius, either increase or decrease the
original central force. In the first case the period of
oscillation is reduced, in the second it is increased.

Now it is easy to determine the light
motion to which this type of motion of the electrons will
lead.

Let us consider first what happens in a
direction running perpendicular to the lines of force. To
the three electron motions there correspond three electrical
oscillations, or in terms of the electromagnetic light theory
three light oscillations of different periods. Thus the light
source will emit three-colour light instead of the
original one-colour light. Therefore, instead of the
single non-polarized spectral line we shall see three separate
lines when we place the light source in a magnetic field. The
different directions of oscillation of the electrons affect the
polarization state of the emitted light. The light of the middle
component oscillates in parallel with, and that of the outer
components perpendicular to the lines of force.

I will presently show you as an
illustration a line which actually displays this behaviour
postulated by Prof. Lorentz's theory.

But let us first consider the rays which
run parallel with the lines of force. For this purpose I
will rotate the model so that the arrow points in your direction.
The opposite circular oscillations of the electrons excite two
circularly polarized rays rotating in opposite directions, one
having a longer and the other a shorter period of oscillation
than the original spectral line. The original spectral line
splits up under the action of the magnetic field into two
components which are circularly polarized in opposite directions.
The light source emits two-colour light.

I would now like to project for you two
enlargements of photographs taken with the aid of Rowland's
grating.* The lines are cadmium lines. In
the first half of the picture you can see the unchanged line, and
in the second rectilinear oscillations occurring in the direction
of the magnetic lines of half the line changed by magnetic
forces, the so-called triplet, which we see in the direction
perpendicular to the lines of force.

Secondly I will project for you a cadmium
line observed in the direction of the lines of force. The first
half of the picture shows the unchanged line, and the second half
the double line or doublet produced by the magnetic forces.

You see how beautifully the consequences
following from Prof. Lorentz's theory were confirmed by
observation in these cases. I should point out, however, that at
first some difficulty was experienced in observing the phenomena
predicted by the theory, owing to the extreme smallness of the
variations in the period of oscillation.

I have just said that the change was
extremely small; but it could be said that it was unexpectedly
large. The magnetic cleavage of the spectral lines is dependent
on the size of the charge of the electron, or, more accurately,
on the ratio between the mass and the charge of the electron. Let
us see what the observations teach us. When Prof. Lorentz
published his theory in 1895, no data were available from which
to estimate the ratio between the mass and the charge of the
light-exciting particles, and in this theory the ratio was left
undefined. We can now calculate this ratio from the magnitude of
the magnetic splitting of the spectral lines: it is
107 c.g.s. units per gram, a colossal number even for
the physicist, since it is 1,000 times as great as the similar
number which was known from electrolysis phenomena in the case of
hydrogen atoms. This makes it most probable for the physicist
that in the luminous particles only ca. 1/1000th of the atom
oscillates, and that the main mass of the atom remains virtually
stationary. The oscillating electrons and the electrolysis ions
were found to be not identical with each other; if they had been,
the splitting of the spectral lines would have been only one
thousandth of that observed, and then I should not have had the
honour of addressing you in Stockholm today.

A further question must also be answered
here and now, namely, are the oscillating particles positively or
negatively charged?

We observed the doublet in the direction of
the magnetic lines of force and studied the sign of the
polarization. Then I suddenly resolved the problem: the
oscillating electrons are negatively charged. We now know
that cathode rays, which can occur in tubes filled with highly
rarefied gases, are negative particles with the same high
charge/mass ratio. We can conclude that that which vibrates in
the light source is the same as that which travels in cathode
rays.

We can hardly avoid recalling the two
titles of Faraday's basic work: "Magnetization of light",
"Illumination of lines of force" They appear to us to be almost
prophesies, because we have now seen that light can in fact be
magnetized, and according to Prof. Arrhenius's theory we have in
nature itself, in the northern lights, an example of illumination
of the magnetic lines of force of the Earth by the electrons
escaping from the sun.

Nature gives us all, including Prof.
Lorentz, surprises. It was very quickly found that there are many
exceptions to the rule of splitting of the lines only into
triplets. The French physicist Cornu was perhaps the first to
observe that, contrary to the elementary theory, in some cases
splitting into four lines, a quadruplet, occurs. In other cases
splitting into five, six or even nine lines can be observed. In
the line-rich spectrum of iron we find a whole selection of
different forms. Very soon a number of physicists were working in
the extended field; I need only name Becquerel, Cotton,
Michelson, Preston, Righi, Runge, and Paschen. If I had more time
at my disposal, I would gladly deal in greater detail with the
work of the last-mentioned investigators. For the present,
however, I must confine myself to projecting a cadmium line for
you, which is split up into four lines, and negatives of a
mercury line which has split up into nine components, and for
which I am grateful to Prof. Runge. But despite this very
complicated splitting-up, even when larger aids are used, the
division into three groups of oscillations, two perpendicular to
and one parallel with the lines of force, assumed in Lorentz's
elementary theory, remains valid, as this photograph of the nonet
shows.

It was natural that, soon after I had
succeeded in splitting up lines, I should also study how the
different lines behave in this respect. I was soon able to show
by investigating the zinc lines that there are great differences
in the splitting-up of different lines of a substance.
Particularly great differences were found in lines belonging to
different series, the discovery of which we owe to the lucid
investigations of your countryman Prof. Rydberg, and in
particular Professors Kayser and Runge.

I found very great differences in the lines
of the different series, and it appeared that the splitting-up,
contrary to the postulations of the elementary theory, expressed
in the scale of oscillation frequencies, is not the same
for all lines in the same magnetic field. We can conclude from
this on the basis of Lorentz's theory that the charge/mass ratio
is not the same for all electrons.

I would now like to talk about three
separate phenomena, first a phenomenon which I have not been able
to observe, secondly phenomena which I have hardly been able to
verify, and thirdly a surprising phenomenon.

All the results which have been discussed
so far have related to line spectra; but in the case of many
bodies we also know of the existence of band spectra. Here a
difference is found : the band spectrum displayed by iodine
vapour or bromine vapour as an absorption medium at low
temperatures, remains unchanged in a magnetic field; I personally
have been unable to bring about any change in the extremely
accurate images which Prof. Hasselberg has given us of the
absorption spectra of bromine and iodine vapour, even with the
strongest magnetic fields.

We are indebted to Prof. Voigt in
Göttingen for a comprehensive theory of magneto-optical
phenomena. The triplet you have seen today was absolutely
symmetrical, as postulated in the elementary theory. Now on the
basis of his theory Prof. Voigt was able to predict that as a
result of the action of weak magnetic forces asymmetry
should occur. According to him, the two external components
should have different light intensities and be at different
distances from the centre line. In the case of iron, zinc, and
cadmium lines I was able to observe both asymmetries; because of
their extreme smallness, however, I cannot demonstrate the
phenomena in the projector.

However, another phenomenon, which will
give you some idea of the scope of Voigt's theory, is more
striking. In this phenomenon Faraday's magnetic rotation of the
polarization plane and the magnetic splitting of the spectral
lines, are intimately connected with each other.

The rotation of the polarization plane is
extraordinarily small in all gases, thus also in sodium vapour.
As Macaluso and Corbino found, it is only in the case of those
colours which lie close to an absorption band of the vapour that
the rotation is very great, of the order of 180°, and the
rotation takes place in the positive direction, the
direction of the current exciting the magnet.

What about the rotation inside the
absorption band?

Prof. Voigt was able to predict that in the
case of highly rarefied vapours the rotation must be negative in
the zone between the two components of the doublet, i.e. opposite
in direction to that outside the band, and also very great. I had
the pleasure of confirming this theoretical finding in
experiments on sodium vapour. Provided that the vapour is highly
rarefied, the rotation in very strong fields between the lines of
the doublet can rise to -400°.

To give you some idea of the distribution
of the rotations, I will show you two negatives connected with
this investigation.

The magnetic field is not set
up.

The two dark vertical lines are the
absorption lines of sodium vapour, the well-known D-lines. The
reason why they are so broad is that the vapour was very dense.
The horizontal bands are interference bands, which were produced
by means of a special device. They indicate the points where the
direction of oscillation is the same. The directions of
oscillation in each of the successive bands differ by
180°.

Now as soon as the magnetic field is set up
we get the image now being projected. On each side of each of the
D-lines the bands bend upwards, the more so the smaller
the distance, because the rotation in the vicinity of the bands
grows very rapidly and reaches almost 180° in the immediate
neighbourhood of the bands. Within the bands a blurred band only
is visible.

The phenomenon becomes far clearer once the
vapour is highly rarefied. The bands bounding the components
rise as before. At the same time, however, the inner band
becomes detached; it has fallen, the rotation is negative.
In our third image the rotation in the case of one of the D-lines
is about -90°, in the other everything is more blurred, the
rotation is about -180°.

Summarizing briefly the results of the
tests described in the light of Lorentz's theory, it can be
stated that firm support has been found for the assertion that
electricity occurs at thousands of points where we at most
conjectured that it was present. Innumerable electrical particles
oscillate in every flame and light source. We can in fact assume
that every heat source is filled with electrons which will
continue to oscillate ceaselessly and indefinitely.

All these electrons leave their impression
on the emitted rays. We can hope that experimental study of the
radiation phenomena, which are exposed to various influences, but
in particular to the effect of magnetism, will provide us with
useful data concerning a new field, that of atomistic astronomy,
as Lodge called it, populated with atoms and electrons instead of
planets and worlds.

I count myself fortunate to be able to
contribute to this work; and the great interest which the Royal
Swedish Academy of Sciences has shown in my work and the
recognition that it has paid to my past successes, convince me
that I am not on the wrong track.

* A
number of lantern slides were projected in the course of the
lecture.